1. Field of the Invention
This invention relates generally to patterned-media perpendicular magnetic recording disks, and more particularly to a method for making the disks.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media, also called bit-patterned media (BPM), have been proposed to increase data density. In BPM the magnetic recording layer on the disk is patterned into small discrete magnetic data islands or “dots” arranged in concentric data tracks. BPM disks may be perpendicular magnetic recording disks, wherein the magnetization directions of the magnetized regions are perpendicular to or out-of-the-plane of the recording layer. To produce the required magnetic isolation of the patterned data islands, the magnetic moment of the spaces between the islands must be destroyed or substantially reduced to render these spaces essentially nonmagnetic.
In one method for forming the discrete magnetic islands, a continuous hard mask layer is formed over a continuous recording layer. A layer of resist is then patterned on the hard mask layer, for example by nanoimprint lithography. The hard mask layer is then etched, using the patterned resist as a mask, to form pillars of hard mask material on the recording layer. The recording layer is then etched, typically by ion-milling, using the hard mask pillars as an etch mask, to replicate the pattern of hard mask pillars onto the underlying recording layer. The hard mask material is removed, leaving the pattern of magnetic islands separated by nonmagnetic spaces or “trenches”.
To achieve recording densities of greater than 1 Terabytes/square inch (Tb/in2), the width of the magnetic islands and the width of the nonmagnetic trenches are critical dimensions that are required to be extremely small, e.g., between 5 and 20 nm, and to have very small tolerances. Thus a challenge to achieve higher areal bit density is in the ability to create uniformly-shaped discrete magnetic islands with narrow trenches between the islands. The physical shape of the hard mask pillars that are formed from the etching process, i.e., their height, width and edge slope, affect the dimensions of the magnetic islands. Optimization of the hard mask pillar geometry together with optimization of the ion-milling process is needed to create a uniform array of magnetic islands. The ion-milling process also erodes the hard mask pillars. However, if some of the hard mask pillars are shorter than desired, or if the etching time is too long, the pillars may be excessively eroded. Also, some of the hard mask material may be re-deposited on the pillars' sidewalls. The hard mask pillar erosion and sidewall re-deposition results in pillars with undesirable shapes, such as large edge slope and large distribution in width, which carries over into the shape of the etched magnetic islands. The non-uniform shape of the magnetic islands translates into poor recording performance of the BPM.
An additional limitation of the prior art method using nanoimprint lithography is that the lateral dimension of the magnetic islands is fixed by the imprint template and cannot be altered. In certain cases it may be desirable to increase the lateral dimension of the magnetic islands during the ion-milling process, for example to decrease the spacing between the islands.
What is needed is a method for making a BPM disk using a patterned hard mask layer that creates uniformly shaped magnetic islands and that also allows the lateral dimension of the magnetic islands to be increased.